1. Field of the Invention
The present invention relates to a touch-down type developing device that, by using a two-component developer composed of a magnetic carrier and a toner, contactlessly develops an electrostatic latent image on an image carrier while holding only a charged toner on a developing roller, and to an image forming apparatus, such as a copier, a facsimile, or a printer, provided therewith.
2. Description of Related Art
Known as conventional developing methods using a dry toner in an image forming apparatus employing an electrophotographic process are: a monocomponent developing method not using a carrier; and a two-component developing method of, by using a two-component developer for charging a non-magnetic toner with a magnetic carrier, developing an electrostatic latent image on an electrostatic latent image carrier (photoconductor) with a magnetic brush of a toner and a carrier formed on a developing roller.
The monocomponent developing method is suitable for achieving higher quality since an electrostatic latent image on an electrostatic latent image carrier is not disturbed by a magnetic brush. On the other hand, with the monocomponent developing method, the layer thickness on a developing roller is regulated by an elastic regulating blade, so that an additive of the toner may adhere to the developing roller, disturbing toner charge and thus making it difficult to maintain a stable charge amount of toner. Moreover, the toner may adhere to the regulating blade and the developing roller, thereby resulting in ununiform layer formation and thus causing image defect. Moreover, since the layer thickness on the developing roller is regulated by the elastic regulating blade, there arisen problems involved in achieving higher speed and longer life, such as lack of durability in the regulating blade due to the achievement of higher speed.
Moreover, for color printing in which colors are superimposed on one another, a toner needs to be non-magnetic since it is required to have permeability. Thus, in a full-color image forming apparatus, the two-component developing method is adopted in many cases in which only a toner not containing a carrier component is charged and conveyed. However, although the two-component developing method permits maintaining a stable charge amount for a long time and thus is suitable for achieving a toner longer life, this method is disadvantageous in image quality due to the aforementioned influence of the magnetic brush.
Suggested as one of means for solving these problems is, for example, as disclosed in Patent Documents 1 and 2, a so-called touch-down developing method of moving a developer onto a developing roller installed not in contact with an electrostatic latent image carrier (photoconductor) by using a magnetic roller, then transferring a toner onto this developing roller to form a thin layer with the nonmagnetic toner, and dispersing the toner to a latent image on the electrostatic latent image carrier (photoconductor) by an alternating electric field.
With this technique, the two-component developing method is adopted for a toner-charged region in view of achieving a toner longer life, while the monocomponent developing method of contactlessly dispersing only the toner to the photoconductor is adopted for subsequent developing regions for the purpose of achieving higher image quality, so that respective advantages of the monocomponent developing method and the two-component developing method can be taken. Moreover, since the toner is charged in an area between the toner and the carrier, an amount of charged toner is relatively stable, which is advantageous in preventing toner dispersion, fog, etc. Thus, this is the most preferable developing method for a full-color image forming apparatus that is required to achieve higher speed, higher image quality, and a longer life.
However, in the touch-down developing method, an increased charge amount of toner of a small grain size accumulates on the developing roller and an electric field for forming a toner thin layer is thereby weakened, which lowers the toner exchange efficiency on the developing roller, thus causing image concentration deterioration and development hysteresis (ghost).
As a method of preventing the image concentration deterioration and the appearance of a ghost, Patent Document 1 discloses a method of making a potential difference of DC bias between the developing roller and the magnetic roller during the first-cycle toner thin layer formation on the developing roller larger than potential differences during the second-cycle and further-cycle toner thin layer formation to thereby ensure a required amount of a toner layer from the first cycle. Moreover, Patent Document 2 discloses a method of setting at zero a potential difference of DC bias between the developing roller and the magnetic roller during non-image formation and exchanging a toner layer on the developing roller with AC bias only.
However, with the method of Patent Document 1, the image concentration deterioration can be suppressed at the start of driving, but it is difficult to suppress the image concentration deterioration over a long period of time. In Patent Document 2, it is required to provide a long gap between sheets to perform toner layer exchange between the sheets during continuous image formation, thus presenting a problem of a deteriorated image formation success rate.    [Patent Document 1] JP-A-2003-21961    [Patent Document 2] JP-A-2003-21966